Recently, the spread of communication services, the advent of various multimedia services, the appearance of high-quality services, etc., have rapidly increased the demand for wireless communication services. To actively meet the demand, it is necessary not only to increase the capacity of the communication system but also to increase the reliability of data transmission.
As a method for increasing the communication capacity in a wireless communication environment, one may consider a method of finding a new available frequency band and a method of increasing the efficiency of given resources. One example of the latter method is a Multiple Input Multiple Output (MIMO) Antenna technology, which is under active development while drawing great attention. According to the MIMO technology, a transmitter/receiver is equipped with multiple antennas to secure a spatial region for utilization of resources to obtain a diversity gain or data is transmitted in parallel through the antennas to increase the transmission capacity.
Generally, MIMO schemes are employed to increase the reliability of the communication system or to improve the transmission efficiency and are mainly classified into three, beamforming, spatial diversity, and spatial multiplexing schemes.
In the beamforming scheme and the spatial diversity scheme using multiple transmitting antennas to increase the reliability, a single data stream is generally transmitted through multiple transmitting antennas. In the spatial multiplexing scheme used to increase the transmission efficiency, multiple data streams are simultaneously transmitted through multiple transmitting antennas.
The number of data streams simultaneously transmitted in the spatial multiplexing scheme is referred to as a spatial multiplexing rate, which must be selected appropriately according to the number of transmitting and receiving antennas and the channel states. Generally, the maximum achievable spatial multiplexing rate is limited by the lower of the number of transmitting antennas and the number of receiving antennas and a lower spatial multiplexing rate is used as the correlation between channels increases.
When the spatial multiplexing scheme is used, a virtual antenna signaling scheme can be applied to achieve a variety of benefits.
For example, if the virtual antenna signaling scheme is applied, channel environments of multiple data streams become identical, thereby making it possible to provide strong channel quality information (CQI) and to increase the reliability of data streams in bad channel conditions.
It is also possible to keep transmission power levels of physical antennas almost equal by applying the virtual antenna signaling scheme to the antennas. More specifically, physical transmitting antennas form a set to create a plurality of beams, which correspond respectively to virtual antennas. Here, different beams are created so that the same power is transmitted from all physical antennas and that the channel characteristics are maintained.
Basically, the total number of created virtual antennas indicates an available spatial diversity or spatial multiplexing rate. This number also indicates the amount of overhead required to measure spatial channels. In the following description, “Mt” represents the number of physical transmitting antennas, “Mr” represents the number of physical receiving antennas, “Me” represents the number of available virtual transmitting antennas, and “M” represents the number of simultaneously transmitted layers. Here, the layers are transmission symbols that have been independently encoded and modulated.
On the other hand, preceding is used as a spatial processing scheme to increase the reliability or transmission efficiency of communication systems. The preceding scheme can be used in a multi-antenna system, regardless of the spatial multiplexing rate. The precoding scheme is typically used to increase the signal to noise ratio (SNR) of channels. Generally, the transmitting end transmits data after multiplying it by a matrix or vector optimal for the current channel environment. For the matrix or vector for multiplication, the transmitting end can receive and use a matrix or vector fed back from a receiving end or can calculate and use an optimal matrix or vector if the transmitting end can determine downlink channel information.
This precoding scheme can be used by integrating it with the virtual antenna signaling scheme. However, the integration will alter the preceding matrix or vector and significantly reduce the system performance unless it creates an appropriate integrated form. The following is an example of this.
FIG. 1 illustrates a conventional structure to which both the preceding scheme and the virtual antenna signaling scheme are applied.
This structure uses a method in which the virtual antenna signaling scheme is applied after the precoding scheme is applied. Specifically, after a preceding matrix is used to increase the average channel SNR, respective data symbols of streams are mixed through virtual antenna signaling before transmission to allow the data streams to undergo the same channel, thereby achieving a virtual antenna signaling gain.
However, this method may reduce the efficiency of the optimal preceding matrix due to the virtual antenna signaling since the virtual antenna signaling is applied after preceding.